The present invention relates to a method and apparatus for measuring a position of a movable member, especially, but not only, a linear position of a movable member.
In fluid transfer and routing systems, valves are extensively used to control delivery and otherwise regulate fluid flow. Frequently, proper valve operation and fast and reliable recognition of valve failures are critical to fluid system reliability.
It is known in the art to detect valve failures by measuring characteristic parameters of the fluid being transported. However, such methods largely tend to be very elaborate and require complex testing and analysis methodologies to ensure accurate detection. Furthermore, implementing such failure detection methods tend to undesirably increase development costs, operational costs, system downtime, and, where relevant, retrofit costs.
It is therefore also known to detect a valve failure in accordance with sensing the position of a valve and comparing it to an expected valve position. However, conventional valve position sensors, such as limit switches and potentiometers have low reliability because of their reliance on electrical contacts which tend to wear and deteriorate relatively quickly. Comparatively reliable sensors, such as rotary variable transformers and linear variable differential transformers are expensive. Other position sensors, such as eddy current sensors, Hall effect sensors, proximity sensors, and the like can only operate in a limited temperature range.
Accordingly, a position sensor representing an improvement over the conventional art, as discussed above, is desirable. In particular, a position sensor that is simple, cost-effective to manufacture and implement, interchangeable, and able to work in a wide range of environments is desirable. A corresponding method for detecting a position of a movable member, such as a valve member, is therefore also desirable. Moreover, it is desirable to provide a dual redundant implementation to improve reliability and fault tolerance.
Therefore, the present invention is generally directed to a position sensor including at least one deflectable member, and, more preferably, at least first and second spaced apart and deflectable members. The first and second deflectable members are positioned so as to allow a movable member, such as a valve stem or the like, to move between them, so as to cause them to deflect laterally by an amount corresponding to a position of the movable member relative to the first and second deflectable members. The position sensor according to the present invention also includes a device for determining the amount that the first and second deflectable members are deflected, and to output a signal based on the determined deflection of the first and second deflectable members. The output signal, according to the present invention, is proportional to the position of the movable member. The first and second deflectable members may be, for example, correspondingly oriented cantilever-mounted members each having a fixed proximal end and a free distal end extending in generally the same direction.
The above-described position sensor is useful as, for example, a valve position sensor, where the movable member is or is associated with a valve spool or the like.
A device for determining the amount that the first and second deflectable members are deflected may include first and second Wheatstone bridges provided on the first and second deflectable members in a known manner so that the first and second Wheatstone bridges output signals corresponding to an amount by which the first and second deflectable members are deflected. In a particular example of the present invention, resistive elements of the first and second Wheatstone bridges are provided on both the first and second deflectable members.
A method for detecting a position of a movable member according to the present invention includes passing the movable member between first and second deflectable members so as to deflect the first and second deflectable members, and determining a position of the movable member according to the extent of deflection of the first and second deflectable members.